Natural gas processing can range from simple treating and conditioning for pipeline delivery to complex operations needed to meet specifications to produce pipeline sales gas or liquefied natural gas (LNG). Removal of acid gas compounds such as carbon dioxide and hydrogen sulfide, as well as removal of organic sulfur compounds, water, and mercury is required to meet end-product specifications and to avoid product blockages in downstream process equipment. The level of treatment that is required varies according to the end-product specifications, as well as local environmental regulations.
In some prior art treatment facilities, mercury is removed by a non-regenerable guard bed. Carbon dioxide and hydrogen sulfide are removed by a solvent process employing a solvent such as an amine. Water may be removed by a molecular sieve dehydration unit. One of the more difficult challenges is to remove carbonyl sulfide (COS) and organic sulfur compounds such as mercaptans, disulfides, and polysulfides.
There are two types of flow schemes that are typically employed to remove these organic sulfur compounds to meet the end-product specifications. One flow scheme is a gas phase treatment, in which a molecular sieve unit is designed to remove the organic sulfur compounds into a regeneration gas stream. The regeneration gas may then be treated by a physical solvent to produce pipeline sales gas or fuel gas and an acid gas stream containing the organic sulfur compounds. This acid gas stream, along with the acid gas generated in the amine unit, is then sent to a sulfur plant, such as one operating a Claus process, for sulfur recovery.
The other flow scheme that has been used to remove organic sulfur compounds is a liquid phase treatment, in which all or most of the carbonyl sulfide, organic sulfur compounds, or both are allowed to pass through a dehydration unit. In a natural gas liquid (NGL) unit, sulfur compounds are expected to be concentrated in the NGL stream. The NGL liquids or an after fractionation stream are then treated specifically for COS removal by a specialty-amine. Mercaptans are removed by a regenerable caustic process in which the mercaptans are converted to liquid hydrocarbon disulfides through use of caustics, such as sodium hydroxide or ammonia. Finally, the liquid is sulfur-polished to a low sulfur concentration by a molecular sieve unit to remove the remaining sulfur content.
The gas phase treatment requires a sulfur plant which is quite costly and therefore, only economically viable when the sulfur level of the natural gas is high. On the other hand, the liquid phase treatment requires an NGL unit, which is only economically viable when the gas is rich in C2, C3 and C4 components. The liquid phase treatment also includes a caustic-based process, which requires disposal of the spent caustic.
If a gas processor receives a feed gas from different supply sources, the feed gas sulfur level, its hydrocarbon contents, or both may vary from time to time, which would mean that neither a sulfur plant nor an NGL unit would be economically viable. Due to the shortcomings of the existing systems, an alternative sulfur capture technology would be desirable.
Therefore, there remains a need for an effective and efficient process for removing sulfur from a natural gas feed stream.